Venovenous extracorporeal membrane oxygenation (VV-ECMO) is a supportive therapy, indicated in case of severe, possibly reversible pulmonary failure, refractory to conventional therapies. Despite advances, morbidity and mortality remain high. Severe neurological complications can occur during ECMO, but their exact etiology is not well understood. It is hypothesized that fast correction of severe hypercapnia, a common indication for venovenous ECMO, may be detrimental for the brain. The supposed mechanism is that fast correction of hypercapnia may result in massive cerebral vasoconstriction and impaired cerebral blood flow (CBF). In this prospective, observational study the aim is to quantify change in CBF during routine initial correction of severe hypercapnia during VV-ECMO. Furthermore, the investigators will record any other hemodynamic changes during VV-ECMO. The hypothesis is that a larger decline in PaCO2 will result in a larger decline of CBF.